A typical prior art head and disk system 10 is illustrated in block form in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
The conventional disk 16 includes substrate 26 of glass or AlMg with an electroless coating of Ni3P that has been highly polished. The thin films 21 on the disk 16 typically include a chromium or chromium alloy underlayer and at least one ferromagnetic layer based on various alloys of cobalt. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer is used to improve wearability and corrosion resistance. Various seed layers, multiple underlayers and laminated magnetic films have all been described in the prior art. The laminated magnetic films have included multiple ferromagnetic layers that are separated by nonmagnetic spacer layers and more recently antiferromagnetic coupling has been proposed. It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer structure. The reduced media noise is believed due to reduced exchange coupling between the magnetic layers. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses from a few Angstroms upward that result in the best decoupling of the magnetic layers and the lowest media noise.
In U.S. Pat. No. 6,280,813 to Carey, et al. a layer structure is described that includes at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic coupling/spacer film. In general, it is said that the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing coupling/spacer film thickness and that the preferred 6 Angstrom thickness of the ruthenium coupling/spacer layer was selected because it corresponds to the first antiferromagnetic coupling peak in the oscillation for the particular thin film structure. Materials that are appropriate for use as the nonferromagnetic coupling/spacer films include ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. Because the magnetic moments of the two antiferromagnetically coupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically coupled films add constructively. An embodiment of the structure includes two ferromagnetic CoPtCrB films, separated by a Ru spacer film having a thickness selected to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. The top ferromagnetic layer is designed to have a greater Mrt than the bottom ferromagnetic layer, so that the net moment in zero applied magnetic field is low, but nonzero. The Carey '813 patent also states that the antiferromagnetic coupling is enhanced by a thin (5 Angstroms) ferromagnetic cobalt interface layer added between the coupling/spacer layer and the top and/or bottom ferromagnetic layers. The patent mentions, but does not elaborate on the use CoCr interface layers.
In U.S. Pat. No. 6,567,236 to Doerner, et al., titled “Antiferromagnetically Coupled Thin Films for Magnetic Recording,” an antiferromagnetically coupled layer structure for magnetic recording wherein the top ferromagnetic structure is a bilayer structure including a relatively thin first sublayer of ferromagnetic material in contact with the coupling/spacer layer. The first sublayer has a higher magnetic moment than the second sublayer. The second sublayer has a lower magnetic moment and is much thicker than the first sublayer with a composition and thickness selected to provide the Mrt when combined with first sublayer that is needed for the overall magnetic structure. A preferred embodiment of a layer structure according to the patent is a pre-seed layer preferably of CrTi; a seed layer preferably of RuAl; an underlayer preferably of CrTi; a bottom ferromagnetic layer preferably of CoCr; an antiferromagnetic coupling/spacer layer preferably of Ru; and a top ferromagnetic structure including: a thin first sublayer of material preferably of CoCr, CoCrB or CoPtCrB, and a thicker second sublayer of material preferably of CoPtCrB with a lower moment than the first sublayer.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains, i.e. the superparamagnetic effect. The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. At some point, as V decreases, the stored magnetic information will no longer be stable under the storage device's operating conditions.
One approach to the solution of this problem is to use a higher anisotropy material, i.e. one with a higher Ku. However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a practical write heads. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
Substantially improved SNR can be achieved by replacing a single magnetic layer with a laminated magnetic layer stack of two (or more) magnetic layers separated by nonmagnetic spacer layers. The reduction in media noise by lamination is believed due to a decoupling of the magnetic exchange coupling between the magnetic layers in the laminate. Spacer layer materials, including Cr, CrV, Mo and Ru, with thicknesses, from 5 to 400 A have been reported to achieve good decoupling of the magnetic layers. Even a discontinuous chromium film has been claimed to reduce the exchange coupling between the two magnetic layers.
Published US patent application 2002/0098390 describes a laminated medium for horizontal magnetic recording that includes an antiferromagnetically (AF)-coupled magnetic layer structure and a conventional single magnetic layer. The AF-coupled magnetic layer structure has a net remanent magnetization-thickness product (Mrt) which is the difference in the Mrt values of its two ferromagnetic films. The type of ferromagnetic material and the thickness values of the ferromagnetic films are chosen so that the net moment in zero applied field will be low, but nonzero. The Mrt for the media is given by the sum of the Mrt of the upper magnetic layer and the Mrt of the AF-coupled layer stack.
The convention for alloy composition used in this application gives the atomic percentage of an element as a subscript; for example, CoCr10 is 10 atomic percent Cr with balance being Co and CoPt11Cr20B7 is 11 atomic percent Pt, 20 atomic percent Cr and 7 atomic percent B with the balance being Co.